1. Field of the Invention
The present invention relates generally to the field of plug valves and more particularly to the field of non-lubricated double block and bleed plug valves.
2. Background Art
The Twin Seal double block and bleed plug valve uses a "retract before turning to open motion" which displays both 1/4 turn rotary and linear movement with multi-turn input. This is presently accomplished by valve operators comprising a two piece stem set which when properly coordinated with the housing, will produce linear travel when the outer stem and housing are locked with the roller preventing rotary motion and allowing only linear motion while the input rotates. Similarly, at the extreme top of the motion of the outer stem, the roller is allowed to move inwardly (radially) locking the inner and outer stem together for only rotary motion (1/4 turn only).
There are presently six basic functions being performed in these operators.
1) INPUT--rotary handwheel or motor input--provides turn force; PA1 2) "L" SLOT--prevents operator from overtravel/undertravel/premature travel/improper travel; PA1 3) TRANSLATION--forces rotary or linear travel; PA1 4) THREADS--provides linear force; PA1 5) INDICATOR--provides visual indication of plug position at the top of the operator; PA1 6) PLUG CONNECTION--connects the operator to the plug.
Such prior art deploys these six functions in numerous discrete locations as can be seen in patented prior art designs. Attempts were made in the prior art to "compact" the stem sets by combining the input and threaded sections. Also a rotary translation device was implemented to assist in minimizing stem length, but this device only allowed for one revolution of the stem (1/4 for turn 3/4 for lift) which limited use to high-pitched stems. Also the stem doubled as the indicator shaft. This reduced the six functions to four discrete locations.
The problem with stem sets having numerous discrete and separate locations for accomplishing the listed six functions, is length. Long operators are inherently more susceptible to damage, less convenient for positioning valves in tight spaces, susceptible to instability and warpage and not conducive to casting with higher strength materials. Moreover, fabrication tends to be more labor intensive thus contributing to a higher price valve.
It would be highly advantageous if it were possible to provide a stem set configuration which significantly reduced the length of the valve operator.